The modern synthesis offers
a powerful paradigm for the evolution of life, but looks still inadequate to
explain the origin of species. The problem is that it is very difficult to
create a new animal. New traits must be assembled in such a way that they allow
the organism to survive at least a few generations. The new traits must
stabilize. The mutating individuals must avoid being rapidly re-assimilated
into the original species through interbreeding. And, last but not least, both
sexes must arise at the same time.

Darwindid not solve the mystery of
the origin of species, despite the fact that he titled his book that way. Only
after the advent of genetics, and mainly thanks to the work of the German
zoologist Ernst Mayr, were biologists able to
advance hypotheses. If a population splits in two because of whatever accident,
both random mutations and environmental differences (natural selection) will
cause the two groups to evolve differently until they become two separate
species.If the two groups ever meet
again, they are more likely to compete than to interbreed, as any species that
have similar behavior in the same territory.

Today, there is consensus
that species are born (at least) from geographic isolation of a population, but
then it is not clear what biological mechanism originates hybrid infertility
(this population cannot inbreed with other populations anymore) and fertile
diploids (this population has both male and female that can breed).

It is not clear whether the
same sudden discontinuity at the level of the phenotype (the organism) occurs
as well at the level of the genotype (its genome). We know that the genotype is
not the same for every member of a species, that small changes occur all the
time. It may well be that changes can accumulate for a long period of time
without any visible consequence on the phenotype while they are reaching a
crisis point. At that point of genetic “drift”, the smallest change in the
genotype may have catastrophic consequences for the phenotype.

In the opinion of Harold
Morowitz, this is the way organisms
acquire new levels of organization, the way they evolve towards more and more
complexity. All of a sudden a “gateway” opens up that leads to a whole new
range of possibilities. For example, a glue that can hold cells together may be
responsible for the sudden appearance of multicellular organisms which in turn
quickly acquired a completely new behavior.

The US linguist Philip
Liebermanbelieves in “functional
branch-points”. He recalls two principles. The first principle is that natural
selection acts on individuals who each vary: species that successfully change
and adapt are able to maintain a stock of varied traits coded in the genes of
the individuals who make up their population. The second principle is the
"mosaic" principle, which holds that parts of the body of an organism
are governed by independent genes. There are no central genes that control the
overall assembly of the body.Given
these principles, a series of small, gradual changes in structure can lead to
an abrupt change in the behavior of the organism; and an abrupt change in behavior
may cause an abrupt change in morphology which causes the formation of a new
species. New species are formed at "functional branch-points".

By surveying “adaptive
radiation” (the spread of species of common ancestry into different niches) and
“evolutionary convergence” (the occupation of the same niche by outcomes of
different adaptive radiation), the US biologist Edward-Osborne Wilsonargued that opportunity is
likely to cause an explosion of species formation.

The problem is that the
genetic mechanism that fosters variation is not well understood. Several
researchers have observed that bacterial cells tend to choose for themselves
advantageous mutations over harmful mutations. Darwinism and modern genetics,
instead, prescribe that mutations must be absolutely random. One possible
explanation that is not in conflict with Darwinism is that, under conditions of
stress, cells generate many more mutations than they would normally do, and of
these mutations the most advantageous survive and are observed. If confirmed,
this would imply that cells know when the survival of their species is in
jeopardy and enter a state of frenzy in which they produce as many mutations as
possible, hoping that at least one will be able to adapt and resolve the
stress. Natural selection is certainly a weak process for evolving species, but
it would be far more effective if it turns out that it is coupled with another
process which is capable of generating a lot of diversity every time evolution
is desirable because of environmental pressure.